1. Field of the Invention
The present invention relates generally to therapeutic molecules which are useful for modulating apoptosis in a target cell or cell population. More particularly, the present invention provides therapeutic agents which inhibit pro-survival molecules and which are capable of inducing or facilitates apoptosis of a target cell or cell population such as cancer cells. The present invention further provides methods for generating or selecting the therapeutic molecules and pharmaceutical compositions comprising the therapeutic molecules.
2. Description of the Prior Art
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country.
Bibliographic details of references provided in this document are listed at the end of the specification.
Cancer is the second leading cause of death in the developed world. Apart from the suffering it causes to patients and their families it is also one of the most expensive diseases to treat (Zhang, Nat Rev Drug Discov 1:101-102, 2002). Accordingly, notwithstanding the toll on human life, if both treatment costs and the cost of reduced economic productivity are considered, the total annual economic burden to society is expected to be in the order of US$200-500 billion by 2010.
Perturbation of programmed cell death (apoptosis) is a central step in the development of many major diseases including cancer. One family of critical regulators of apoptosis is the Bcl-2 protein family. Studies have shown that Bcl-2 overexpression, enforced in human follicular lymphoma, inhibits apoptosis and contributes to tumorigenesis (Vaux et al., Nature 335:440-442, 1988; and Strasser et al., Nature 348:331-333, 1990). Bcl-2 overexpression has also been noted in up to 90% of breast, colonic and prostatic cancers (Zhang, 2002, Supra), which represent some of the most common cancers. Pro-survival relatives of Bcl-2 are also overexpressed in many tumors. Indeed, impaired apoptosis is now accepted as a central step in the development of most forms of malignancy (Cory et al., Nat Rev Cancer 2:647-656, 2002).
Impaired apoptosis is also a major impediment to the efficacy of cytotoxic cancer therapy (Cory et al., 2002, Supra; Johnstone et al., Cell 108:153-164, 2002). Most cytotoxic agents, including many chemotherapeutic drugs and radiation, indirectly trigger apoptosis through molecules such as the tumor suppressor p53 (Cory et al., 2002, Supra). In most tumors, however, the p53 pathway is inactivated, preventing the signals to initiate apoptosis. Hence, either loss of p53 function or overexpression of Bcl-2 can provoke chemoresistance, a common cause for treatment failure.
Those members of the Bcl-2 protein family that promote cell survival, including mammalian Bcl-2, Bcl-xL, Bcl-w, Mcl-1 and A1, contain three or four BH (Bcl-2 homology) regions of sequence similarity, and function until neutralized by their BH3-only relatives. These pro-apoptotic antagonists, which include mammalian Bim, Puma, Bmf, Bad, Bik, Hrk, Bid and Noxa, are related to each other and the wider family only by the short BH3 domain (Huang and Strasser, Cell 103:839-842, 2000). In contrast, Bax and Bak, a sub-group of pro-apoptotic family members, share three BH domains with Bcl-2 and have an essential downstream role, probably in permeabilization of intracellular membranes (Wei et al., Science 292:727-730, 2001).
The BH3-only proteins monitor cellular well-being and damage signals trigger their binding to pro-survival Bcl-2-like proteins, thereby initiating cell death (Cory et al., Oncogene 22:8590-8607, 2003; Huang and Strasser, 2000, Supra). Their differential activation, induced by transcriptional cues (e.g. Bim, Puma, Noxa) or various post-translational mechanisms (e.g. Bim, Bmf, Bad, Bid), imparts some signaling specificity (Puthalakath et al., Cell Death Differ 9:505-512, 2002). Once activated, however, the various BH3-only proteins are generally thought to function similarly by targeting all the pro-survival Bcl-2-like proteins (Adams et al., Genes Dev 17:2481-2495, 2003; Cory et al. 2003 supra; Huang and Strasser, 2000, Supra). Until recently their interactions have not been systematically characterized, and the few quantitative studies were confined to Bcl-xL or Bcl-2 (Letai et al., Cancer Cell 2:183-192, 2002; Petros et al., 2000, Supra; Sattler et al., 1997, Supra). A thorough study has now been published (Chen et al. Mol Cell 17:393-403, 2005) revealing that some of the BH3 only proteins are promiscuous binders and others are more selective. Establishing whether the diverse BH3-only proteins and pro-survival family members interact selectively or promiscuously is important for clarifying how cell death initiates (Adams, 2003, Supra; Cory et al., 2003, Supra; Danial and Korsmeyer, Cell 116:205-219, 2004) and is very pertinent to current efforts to develop compounds that mimic the action of BH3-only proteins as novel anti-cancer agents.
In light of the requirement for less toxic and better targeted anti-cancer therapies, there is a clear need for the identification of molecules which interact with Bcl-2-like proteins to inhibit their pro-survival function.